Hi, I have to write ParseChart implementation with Data.Map/Set. The chart is type like this: type Chart k v = Map k (Set v) now I need operation like: insert :: k -> v -> Chart k v -> Maybe (Chart k v) where the result is (Just _) if the (k,v) is actually added to the chart or Nothing if it was already there and nothing have to be done. The straight forward implementation is: case Map.lookup k chart of Nothing -> Just (Map.insert k (Set.singleton v) chart) Just set | Set.member v set -> Nothing | otherwise -> Just (Map.insert k (Set.insert v set) chart) The problem with this is that both the Map and the Set are traversed twice. The first time from lookup/member and the second time from insert. Does someone have an idea how to do this with the current libraries? There are the Map.updateLookupWithKey and the Map.alter functions: updateLookupWithKey :: Ord k => (k -> a -> Maybe a) -> k -> Map k a -> (Maybe a,Map k a) alter :: Ord k => (Maybe a -> Maybe a) -> k -> Map k a -> Map k a which are the closest that I need. The problem is that the first doesn't allow the client function to be called if there isn't matching key in the map and the second is doesn't allow to return value from the client function. What I really need is an alterLookúp function: alterLookúp :: Ord k => (Maybe a -> (b,Maybe a)) -> k -> Map k a -> (b,Map k a) The chart manipulation is in the tight loop of my application so I need fast code. Any other ideas? Regards, Krasimir